ウマ娘キャラ診断 means Uma Musume character diagnosis, an online diagnosis test that tells you which Uma Musume/horse you are.

As mentioned in a post before,

…Horse racing is quite popular in Japanese society (partly due to the gamble/lottery related), and inspired subculture projects such as the phenomenal ウマ娘 プリティーダービー (Uma Musume Pretty Derby)…

Today (May 12), an online test becomes viral on Twitter in Japan. Users post the result of which horse they have been diagnosed as on Twitter, with the tag #ウマ娘キャラ診断 or #あなたをウマ娘のキャラに例えるなら, which causes them trending.

Such online tests are popular all over the world, with ambivalent questions and iffy results. Notwithstanding, they could be somewhat of an online small talk and create topics for discussion.

Learn more on wikipeida about Uma Musume Pretty Derby.